dora-test-utils: Bringing Unit and Integration Testing to DORA

DORA lacks testing infrastructure. Debugging multi‑node dataflows means manual node isolation—slow and fragile. This project delivers `dora-test-utils`: a Rust crate that adds unit testing (NodeHarness), integration testing (TestSourceNode/TestSinkNode), and optional record/replay for regression. Deliverables include mock APIs, example pipelines, documentation, and CI templates. The work is scoped as a 175h MVP with a +175h extended version.